Exiv2 modify action always writes from metadata

The modify action of the exiv2 utility should use the non-intrusive writing logic whenever possible. Instead, it always writes from metadata, independent of the modification.

Additional information:

Modify::clone uses the half-cooked ExifData copy construction which sets ExifData::compatible_ to false with the described effect.Replace the ExifData (and IptcData) members of Modify with pointers to avoid using ExifData copy construction or assignment.